Abrasion-resistant agglomerate mineral substance granule, powder comprising such granules and production method therefor

ABSTRACT

Abrasion-resistant granules of an agglomerated mineral substance, powder comprising such granules and process for manufacturing it Granule comprising an agglomerate of particles of a mineral substance. The agglomerate is coated with a monolithic outer layer, which provides it with improved abrasion resistance.  
     Powder comprising granules according to the invention.  
     Process for manufacturing the powder according to the invention, which includes an agglomeration step and a coating step.

[0001] The invention relates to abrasion-resistant granules of anagglomerated mineral substance. The invention also relates to a powderformed from such granules, to its use and to a process for manufacturingit.

[0002] Many mineral substances, such as for example sodium or magnesiumsalts, are often used in the form of granules.

[0003] For these granules to be used effectively, they must in generalhave an appropriate size. For example, for many applications, such asadditives for detergents or for medical or pharmaceutical uses, the meandiameter of the granules is ideally between 250 and 1000 microns.

[0004] An important characteristic of the techniques for producinggranules is their particle size distribution. Such techniques result inremarkably narrow distributions. Unfortunately, they are generally moreexpensive. If it is desired to have a technique that operateseconomically, the particle size distribution is usually very wide, whichhas the consequence that the size of all the granules obtained is rarelysatisfactory for a given application. Under these conditions, it isknown to separate the granules according to their size, for example byscreening, so as to assign them to different applications.

[0005] In order for all that produced to be used, it would be necessaryfor the consumption of the various particle size classes to correspondto the quantities produced. This is rarely the case and there is asurplus, the quantities with fine particle sizes often being in excessover those with coarse particle sizes.

[0006] In addition, when the granules are produced by crystallizers, theproductivity (expressed as tons/hour for example) decreasessubstantially when it is desired to produce coarse-diameter granules.Concomitantly, their manufacturing cost obviously increases.

[0007] To solve this problem, it is known to agglomerate fine particlesof a mineral substance in order to obtain coarser ones. Mechanicalagglomeration techniques such as compacting can be used. However, theseagglomerated products have the disadvantage of being friable. As aresult of their poor abrasion resistance, fine particles reappear whenhandling them. In addition, these known agglomeration techniques areapplicable only to mineral substances that agglomerate easily. Forothers, the friability of these known granules is such that they aredifficult to use.

[0008] It has been attempted to remedy this drawback by using anionicsurfactant additives during the manufacture of the agglomerates. Such ause is described, for example, in European Patent Application EP 0 452164 which relates to sodium perborate tetrahydrate with improvedabrasion resistance, formed from separate particle agglomerates. Thedimensions of the said particles do not exceed 30 um. However, the useof additives is a drawback for the production of granules of a mineralsubstance when the latter has to be of high purity. In particular,pharmaceutical applications require such high-purity products.

[0009] The invention therefore aims to remedy these drawbacks byproviding granules of an agglomerated mineral substance, which exhibitgood abrasion resistance and can be produced by a process that is simpleand economic and does not require the use of any additive.

[0010] Consequently, the invention relates to a granule comprising anagglomerate of particles of a mineral substance, which is characterizedin that the agglomerate is coated with a monolithic outer layer.

[0011] The granule according to the invention may be of any shape. Itmay have a small diameter or a larger diameter. The economic benefit ofthe invention will, however, be more pronounced in the case oflarge-diameter granules. This is because the production cost of granulesaccording to the invention increases more slowly with the diametersproduced than these of the known granules.

[0012] It is advantageous for the granule according to the invention tohave a mean diameter between 200 μm and 5000 μm. Preferably, its meandiameter is greater than 250 μm. Preferably, it is less than 2500 μm.When the granule is approximately spherical, this mean diameter is itsdiameter. When the granule is of any shape, its mean diameter is definedas being six times the ratio of its volume to its external surface area.

[0013] The mineral substance to which the invention applies may be anymineral substance from which it is desired to form granules from smallerparticles. As an example, mention may be made of: sodium or magnesiumchlorides, sodium carbonate and sodium bicarbonate. It is advantageousto apply the invention to mineral substances that give rise to particleagglomerates having poor mechanical properties.

[0014] The invention is particularly well suited to sodium bicarbonate.

[0015] Depending on its mean diameter and the size of the particles fromwhich it is formed, the agglomerate contains a variable number of theseparticles, namely from a few tens of particles to several million.

[0016] According to the invention, the agglomerate is coated with amonolithic outer layer. The term “monolithic layer” is understood tomean a layer of material that is essentially continuous. It is notagglomerated. The material from which it is formed is essentially madeas a single unitary, non-agglomerated block. It is preferably in acrystalline state. The layer may be a single crystal. More generally, itis a polycrystal. The monolithic layer forms an envelope around theagglomerate, the function of which is to mechanically retain theagglomerated particles. It may be impermeable or porous and havedefects, provided that they fulfil its function. The monolithic layerthat coats the agglomerate may in certain places also penetrate moredeeply into it, by infiltration of cracks initially present in theagglomerate.

[0017] The thickness of the coating layer must be sufficient forfulfilling its technical function defined above and for providing therequired good mechanical properties. It is unnecessary for its thicknessto be too great, as this would reduce the economic advantages of theinvention.

[0018] The minimum thickness of the monolithic layer depends on variousparameters, among which are, in particular, the substance from which itis formed, the size of the agglomerate, the mean diameter of theconstituent particles and the desired mechanical strength. As a generalrule, it must be determined in each particular case by a routinelaboratory study.

[0019] The monolithic outer layer generally has a thickness of greaterthan 0.25 μm. It is generally advantageous for this layer not to exceed50 μm. It has been observed that monolithic outer layers having athickness of greater than 0.5 μm are particularly suitable. Preferably,their thickness is less than or equal to 30 μm.

[0020] According to the invention, coating the agglomerates with themonolithic outer layer substantially improves their abrasion resistance.

[0021] The choice of constituent material of the outer coating layer isdetermined by the final properties desired. In addition to abrasionresistance, antistatic or hydrophobic properties may, for example, besought by using a coating layer of organic material.

[0022] However, it is advantageous for the monolithic outer layer to beformed from a mineral substance.

[0023] Preferably, the mineral substance of the monolithic outer layeris substantially identical to the mineral substance of the particles.This embodiment has the advantage that the resulting granule may be ofhigh purity.

[0024] In a preferred embodiment of the invention, the granule containsno additives, such as binders or surfactants. In a preferred version ofthis embodiment, the granule contains no binder. Such granules may meetthe strictest purity requirements, whether in the field of humanfoodstuffs or in the pharmaceutical field.

[0025] The invention also relates to a powder comprising a number ofgranules according to the invention described above.

[0026] The powder according to the invention may be formed exclusivelyfrom granules according to the invention. It may also include othergranules, in variable proportions.

[0027] The width of the granule diameter distribution may vary greatly,since it depends essentially on the agglomeration technique used.

[0028] The powder according to the invention can be applied in varioustechnical fields. In particular, powders according to the invention,comprising sodium bicarbonate granules, can be applied for themanufacture of detergents or pharmaceutical products.

[0029] The invention also relates to a process for manufacturing thepowder according to the invention, in which, in a first step,agglomerates of particles of a mineral substance are formed, whichprocess is characterized in that, in a second step, the agglomerates arecoated with a monolithic layer.

[0030] The function of the first step of the process according to theinvention is to agglomerate particles whose mean diameter is too smallfor the intended applications, in order to form agglomerates whose meandiameter is suitable for these applications. Any known agglomerationtechnique may be used-for example, sintering, palletizing or compacting.

[0031] As indicated above, depending on the mineral substance from whichthey are formed and on the technique used to agglomerate them, theparticles adhere to a greater or lesser extent to one another. However,a minimum level of adhesion is necessary in order to ensure cohesion ofthe agglomerate before it is coated.

[0032] In a particular method of implementing the process according tothe invention, the particle agglomerates are formed by compacting. Anysuitable compacting technique may be used. In an advantageous version ofthis method of implementation, the particles are compacted bycompression between two rolls. The cake obtained is then crushed inorder to obtain the agglomerates of desired size. The product obtainedfrom the crushing is screened, excessively fine agglomerates beingrecycled into the compacting operation.

[0033] In the second step of the method according to the invention, theagglomerates are coated with a monolithic layer. The choice of coatingtechnique used (for example, dusting, spraying or immersion) depends onthe precise circumstances of application of the process according to theinvention.

[0034] In an advantageous version of the process according to theinvention, to coat the agglomerates a supersaturated solution of aconstituent substance of the monolithic layer is made to pass through abed of the agglomerates. The bed may or may not be fluidized. However,fluidized beds are preferred. Crystal growth by passing a supersaturatedsolution through a fluidized bed has been described in document EP 0 352847 (SOLVAY SA). For its application to the process according to theinvention, the bed is formed from the agglomerates to be coated. Thesupersaturation of the saturated solution may be obtained by cooling thelatter before it comes into contact with the bed.

[0035] In the implementation version in which the monolithic layer andthe agglomerates are formed from sodium bicarbonate, it has been foundthat the temperature of the bed is advantageously above 30° C. However,it is not worthwhile for this temperature to exceed 70° C.

[0036] In a preferred way of implementing this version, the temperatureof the bed is above 40° C. and below 60° C.

[0037] In another advantageous version of the process according to theinvention, in order to coat the agglomerates, a supersaturated solutionof a constituent substance of the monolithic layer is sprayed onto them.In a preferred way of implementing this version, the agglomerates areintroduced continuously into an inclined rotating drum which is sprayedwith the supersaturated solution. The movement of the drum ensureshomogeneous distribution of the monolithic layer around the agglomerate.After a sufficient residence time (that depends in particular on thedesired thickness of the coating layer and may be set by theinclination, the dimensions and the speed of rotation of the drum), thecoated agglomerates are removed from the drum.

[0038] The invention is illustrated by the following description withreference to the appended drawing.

[0039] The single figure represents the diagram of a plant for carryingout one particular way of implementing the process according to theinvention.

[0040] The plant shown schematically in the figure, the description ofthe operation of which follows, comprises a roll compactor 2, a crusher4, a screen unit 6, a saturation tank 8, a heat exchanger 10, afluidized-bed crystallizer 12 and a dryer 14.

[0041] Sodium bicarbonate particles 1 are introduced into the rollcompactor 2. A compacted cake 3 is produced therein. The cake 3 iscrushed in the crusher 4 into agglomerates 5. A fraction 7, havingparticle sizes between 500 and 1000 μM, is selected from theagglomerates 5 by means of a screen unit 6. The undersize 16 havingdimensions of less than 500 μm is recycled into the compactor 2, whereasthe oversize 17 having dimensions of greater than 1000 μm is recycledinto the crusher 4. The selected agglomerates 7 are introduced into thecrystallizer 12, the fluidized bed of which is formed by saidagglomerates. A saturated sodium bicarbonate solution 9 is produced inthe saturation tank 8. This solution is then cooled in the heatexchanger 10 so as to produce a supersaturated sodium bicarbonatesolution 11. The supersaturated solution 11 is introduced into thecrystallizer 12, where it fluidizes the bed of crystals. The solution 11is desupersaturated on contact with the crystals and the latter are thusprogressively coated with a monolithic layer of sodium bicarbonate. Thewet, coated bicarbonate 13, collected from the crystallizer 12, is driedin the dryer 14 in order to supply the final product 15.

[0042] In a preferred embodiment of the plant shown in the figure, thesaturation tank 8, the exchanger 10 and the crystallizer 12 are combinedinto a single apparatus, of the type described in European Patent 0 352847 (SOLVAY SA).

[0043] The examples, the description of which will follow, bring out thebenefit of the invention.

EXAMPLE 1 Not According to the Invention

[0044] Compacted and crushed sodium bicarbonate agglomerates, theparticle size of which was between 500 and 1000 μm, were subjected tothe following mechanical abrasion test.

[0045] 100 g of granules were placed in a rotating cylindrical drumhaving an inside diameter of 57 mm and a length of 120 mm, in thepresence of 400 g of lead beads 6 mm in diameter. The drum was thenrotated at 140 rpm for 30 minutes. After the treatment, an “abrasionresistance index” was measured, this being defined as the fraction, inpercent by weight, of the content of the drum that passes through a 63μm sieve. The result of the test was 3%.

EXAMPLE 2 According to the Invention

[0046] A bed of 100 g of compacted and crushed sodium bicarbonateagglomerates, the particle size of which was between 500 and 1000 μm,was placed on the fluidization mesh of a fluidization column, the columnbeing in contact with a thermostatic bath whose temperature was set at50° C. The bed was fluidized by the ascending flow, at a velocity of 154m/h, of a supersaturated sodium bicarbonate solution through thefluidization mesh. The supersaturation of the solution was set at 2.4g/kg thanks to a temperature drop of 1.9° C. of the solution through theexchanger.

[0047] The coating of the agglomerates was continued for one hour, afterwhich the granules were collected and the mean thickness of the coatinglayer was measured, namely 3 μm. The granules according to the inventionproduced in this way were then subjected to the abrasion test definedabove in Example 1. The result was 0.6%, demonstrating the veryconsiderable improvement in their abrasion resistance as a result of thecoating according to the invention.

EXAMPLE 3 According to the Invention

[0048] The procedure for this example was as in Example 2, except thatthe supersaturation was set at 4.9 g/kg, after which a 10 μm coatinglayer was deposited on agglomerates. The result of the abrasion test was0.1%, again showing an improvement in the abrasion resistance of thegranules according to the invention.

1-10. (Cancelled).
 11. A granule comprising an agglomerate of particlesof a mineral substance, wherein the agglomerate is coated with amonolithic outer layer.
 12. A granule according to claim 11, wherein themonolithic outer layer is formed from a mineral substance.
 13. A granuleaccording to claim 12, wherein the mineral substance of the monolithicouter layer is substantially identical to the mineral substance of theparticles.
 14. A granule according to claim 11, wherein the mineralsubstance is sodium bicarbonate.
 15. A granule according to claim 11,wherein the granule contains no binder.
 16. A granule according to claim11, wherein the monolithic outer layer has a thickness of between 0.25and 50 μm.
 17. A powder comprising granules according to claim
 11. 18. Aprocess for manufacturing a powder according to claim 17, comprisingforming agglomerates of particles of a mineral substance and coating theagglomerates with a monolithic layer.
 19. A process according to claim18, wherein the particle agglomerates are formed by compacting.
 20. Aprocess according to claim 18, where the agglomerates are coated bypassing a supersaturated solution of a constituent substance of themonolithic layer through a bed of the agglomerates.
 21. A processaccording to claim 19, wherein the agglomerates are coated by passing asupersaturated solution of a constituent substance of the monolithiclayer through a bed of the agglomerates.
 22. A granule according toclaim 12, wherein the mineral substance is sodium bicarbonate.
 23. Agranule according to claim 13, wherein the mineral substance is sodiumbicarbonate.
 24. A granule according to claim 12, wherein the granulecontains no binder.
 25. A granule according to claim 13, wherein thegranule contains no binder.
 26. A granule according to claim 12, whereinthe monolithic outer layer has a thickness of between 0.25 and 50 μm.27. A granule according to claim 13, wherein the monolithic outer layerhas a thickness of between 0.25 and 50 μm.